[Answer]-Why Django created stacked element for custom through model

Django suggests using inlines when you specify a through model because often the intermediate table has extra fields, which couldn’t be represented in a multiple select box.

In your example, you don’t have any extra fields, so the easiest thing to do would be to not specify the through model. Then you can use the multiple select widget.

You might have to use the through model, for example you might be using a legacy database. Unfortunately, I’m not aware of a quick way to enable the multiple select widget in this case.